The present invention relates to a cardiac pacemaker with two unipolar electrodes, at least one indifferent electrode arranged some distance away from these two unipolar electrodes, a series-connected amplifier, filter and detector for each unipolar electrode, and devices for controlling the stimulation pulses for the heart.
In a cardiac pacemaker of this type, the electrodes can be placed in the atrium and in the ventricle of the heart. The indifferent electrode is formed, for example, by the capsule wall of the pacemaker. Such an electrode system with unipolar electrodes has excellent stimulation and detection characteristics. However, with a system of this type, there is the danger that the smooth functioning of the pacemaker will be impaired by spurious signals caused by muscle spasms or by signals originating outside the body. In order to prevent the signals caused by muscle spasms in the vicinity of the pacemaker capsule from being detected by the pacemaker as cardiac signals, a known method is to insulate nearly the entire capsule and to leave only a small conductive area free of insulation which is faced away from the muscle tissue during implantation. However, this procedure reduces the conductive surface of the indifferent electrode and can result in polarization problems.